You finish planing a panel. The surface is flat, the light skips clean across it, and your hand is already reaching for the sandpaper. It is the most automatic move in the shop. Sand to 220 before the finish goes on. Everybody says so.
Hold the hand for a second. Ask one question before it lands. What finish is going on first?
That question is the whole post, because the answer changes whether sanding helps you or hurts you, and the standard advice never sorts it out.
Here is what sanding actually does to the wood. A sharp plane severs the surface fibers and stops there. The damage is one cell layer deep, and the openings in the wood, the vessels and the rays, stay clean and open. Sanding works differently. It tears rather than cuts, and the damage goes down with every coarser grit. Even 120 reaches a few cells deep. Eighty goes deeper still. Worse, the grit packs fine dust into those same vessel openings. You can run a roughness gauge over a planed board and a sanded board and read nearly the same number, but the number is lying to you. It cannot tell a clean open pore from one stuffed with dust. The finish can.
That is the sentence to keep. The pores fill with dust. Oil can't get in where dust already is.
So take the two finishes one at a time.
A film finish sits on top of the wood. Varnish, lacquer, waterborne. It needs something to grip, a little mechanical tooth, and a perfectly planed surface does not always give it that. Plane at a high cutting angle with the chip breaker set tight and you can burnish the surface slightly, press the top fibers down smooth instead of cutting them clean. A film laid over that can let go later. This is the case the old advice was built for. Light sanding at 180 or 220 raises just enough tooth for the first coat to hold. Sand here. It helps.
A penetrating finish is the opposite case. Oil, oil-varnish blends, anything that has to soak in to do its job. It needs those vessel openings clear. The planed surface, clean pores and all, takes oil more evenly and more predictably than a sanded one. Run sandpaper over it first and you pack the pores with dust, and the oil goes in unevenly where it goes in at all. That blotchy oil finish people blame on the wood is often a preparation problem instead. White oak shows it plainly. Those big open pores either drink the oil evenly or they don't, and dust in the pores is what decides it.
So the protocol is two decisions, not one. Plane to your final surface. Film finish going on? Scuff at 180 to 220 for tooth. Oil finish going on? Go straight to the oil, and only touch it with paper if the surface looks burnished, and then just enough to break the sheen. Never run a heavy progressive sanding schedule on a planed board that's headed under oil. You would be undoing the clean surface you just spent an hour making.
The instinct to sand isn't wrong. It just isn't sorted. Do the sort first, then reach for the paper, or don't.